1. Field of the Invention
The present invention relates generally to mobile communications systems.
2. Description of the Related Art
In modern communication systems, the term "mobile" typically refers to a radio terminal that is attached or carried in a high-speed mobile platform (e.g., a cellular telephone in a moving vehicle). This is in contrast to the term "portable" which typically refers to a hand-held radio that is used by a person at walking speed (e.g., a cordless telephone inside a home).
Subscribers generally pay a fee to use a mobile communication system such as the exemplary system 20 of FIG. 1. The system 20 has one or more satellites 22 in orbits (e.g., geostationary) about the earth 24. The satellites 22 have transmit-receive communication links 26 (e.g., at L-band frequencies) with various mobile units 28. The satellites 22 also have communications (e.g., at optical wavelengths) through links 30 to other satellites and links 32 with various system ports that form "gateways" into independent communication systems.
Exemplary gateways are a public-switched telephone network (PSTN) 34 and an access port 36 of a cellular telephone system. The PSTN 34 allows any of the mobile units 28 to communicate over telephone lines 38 with telephone system customers 40. The access port 36 of the cellular system typically has a plurality of transceivers arranged in a cellular network so that communications from the satellites 22 can be transmitted over cellular links 42 to mobile users 44 of the cellular system.
Communications with the mobile units 28 are subject to fading which is a temporary random decrease in the received signal level. The principal types of fading are multipath fading and power fading. Multipath fading occurs in areas where multiple reflected signals (e.g., from nearby buildings) arrive at the mobile unit. The combined power of these signals can vary widely because it is dependent upon the phasing of the signals.
Power fading is caused by blockage of the transmitted signal by fixed structures (e.g., trees and buildings). The received power of a mobile unit can drop dramatically (e.g., on the order of 10 or 20 dB) as it moves into the transmission shadow of such structures. Accordingly, this type of fading is also referred to as shadowing.
FIG. 2 illustrates a typical scenario 50 of power fading. A mobile unit 52 receives signals from a satellite 54 as the unit moves along a road 56 which borders a line of trees 58. Initially, the mobile unit receives signals along a transmission path 60 which is not blocked by the trees. At a subsequent point along the road 56, the mobile unit receives signals along a transmission path 62 which must pass through the canopy of the trees 58. As a result, the signal power at the mobile unit fades. Typically, fading increases with decrease in the elevation angle 64 of the satellite's transmission path.
Extensive studies of shadowing (e.g., see Goldhirsh, Julius, et al., Propagation Effects for Land Mobile Satellite Systems, National Aeronautics and Space Administration Reference Publication 1274, February, 1992) have documented the degree of tree shadowing in different scenarios. As might be expected, the shadowing depends upon a variety of factors such as the tree density, the type of trees, the season and the elevation angle of the transmission path.
Availability is a term often used to define a communication system's reliability. In particular, availability is the fraction of transmission time that communication signals are successfully received. Although an availability of 100% is seldom achieved, this is a goal of communication system design.
In a conventional passive process for improving availability, transmitted signal power is increased by a link margin L.sub.mg above a predetermined power threshold P.sub.th that is necessary for successful reception (e.g., see Robert G. Winch., Telecommunication Transmission Systems, McGraw-Hill, Inc, New York, 1993, pp. 182-186). Although the use of a significant link margin can improve availability in areas subject to heavy shadowing, it also increases the transmission energy and, hence, the transmission cost of the communication system. The cost of satellite-based communication systems are especially sensitive to increases in link margin.
A more efficient approach is an active system that is conventionally referred to as power control. In power control, transmitted power is increased over the power threshold P.sub.th by a static power margin M that is reduced from the link margin L.sub.mg. When a fade in received power exceeds the static power margin M, the transmitted power is temporarily increased by a boost B. The boost B is then removed in response to the fade's termination. Typically, boost removal is delayed from fade termination by a hold time T.
In comparison to passive processes, active power control allows transmitted power to be reduced generally by L.sub.mg -M with temporary power increases of B applied in response to excessive fading. In this process, additional transmitted power is directed only to those system users that are experiencing fading. Because the number of users experiencing fades is generally smaller than the total number of users, power control systems can improve the ratio of availability to cost.
Although conventional power control techniques facilitate an increase in system efficiency, they are generally applied without any means for assessing their efficiency nor any means for determining parameter selections that would further enhance that efficiency.